Sialidase is a glycohydrolytic enzyme present in living bodies, which eliminates a sialic acid residue from a non-reducing terminal of saccharide chains of glycoproteins or glycolipids. It has been known that, when sialic acid is removed from saccharide chain molecules, not only the degradation of these molecules begins, but also molecular conformation and many of important cell functions such as recognition mechanism by receptors, cell adhesion, and immunomechanism may be changed. It has also become clear that sialidase exhibits rapid activity change in connection with proliferation and canceration of cells, and it is involved in the metastatic ability of cancer cells. However, there is little knowledge about how sialic acid is eliminated in vivo. This is because the studies of mammalian sialidases on the molecular level are behindhand, and hence there are many unknown points concerning their structures and expression mechanism.
Because mammalian sialidase exhibits only low activity, and is extremely unstable, isolation and purification of the enzyme have been difficult. Sialidase has been often considered for a long time to be one of the mere lysosomal enzymes involved in the dissimilation and degradation. Under such a situation, we isolated and purified the enzyme by using mainly rat tissues as the source of the enzyme, and found that there were four types of sialidases which differ from sialidases of bacteria, viruses, protozoa and the like in their natures (Miyagi, T. and Tsuiki, S., Eur. J. Biochem. 141, 75-81, 1984; Miyagi, T. et al., J. Biochem. 107, 787-793, 1990; Miyagi, T. and Tsuiki, S., J. Biol. Chem. 260, 6710-6716, 1985). These enzymes each localize in lysosomal matrix, lysosome membrane, plasma membrane (cell surface membrane), and cytoplasm within a cell, respectively, and they are different from each other not only in enzymological characteristics such as substrate specificity, but also in immunological properties. Among those sialidases, the sialidase localized in cytoplasm can be obtained as a homogenous purified product from rat skeletal muscles. Its cDNA cloning has been succeeded for the first time in the world as for animal sialidases, and its primary structure has been determined (Miyagi T. et al., J. Biol. Chem., 268, 26435-26440, 1993). Its genomic structure analysis has also been done, and as for its function, it has been elucidated that the enzyme participates in the differentiation and the growth of skeletal muscle cells by using the cDNA as a probe. These studies can be considered a part of pioneer researches in sialidase studies in the world.
By the previous studies, it has become clear that there is possibility that the sialidase localized in plasma membrane exhibits activity elevation upon proliferation and canceration of cells, and it is also deeply concerned with the differentiation of nerve cells and the signal transduction of cells. To date, however, it has not been understood at all about the structure of this enzyme, the mechanism causing the activity change and the like. In order to answer these questions, what many researchers in this field have long been desired is cloning of its cDNA. For example, if the mechanism of cancerous change due to this enzyme could be elucidated, it would be possible to utilize the results in diagnosis and therapy of cancers. Further, because gangliosides exist in surface membranes of many cells and participate in important cell functions such as cell adhesion and informational communication, and they are also main cerebral components, the sialidase utilizing them as a specific substrate is estimated to be involved in certain important cranial nerve functions.